Conventional golf balls have primarily two functional components: the core and the cover. One purpose of the core is to be the “spring” of the ball or the principal source of resiliency, and the core may be either solid or wound. The primary purpose of the cover is to protect the core. Multi-layer solid balls include multi-layer core constructions, multi-layer cover constructions and combinations thereof. In a golf ball with a multi-layer core, the principal source of resiliency is the multi-layer core. In a golf ball with a multi-layer cover and single-layer core, the principal source of resiliency is the single-layer core.
Two-layer solid balls are made with a single, solid core. This single core is typically constructed from a cross-linked rubber, for example polybutadiene, and is encased by a single layer of hard cover material. Increasing the cross-link density of this core material increases the resiliency of the core. As the resiliency increases, however, the compression may also increase, resulting in a stiffer ball and increasing the spin rate of the ball. Spin rate is an important characteristic of golf balls for both skilled and recreational golfers.
Higher spin rates, although allowing a more skilled player to maximize control of the golf ball, because golf balls to have severely parabolic trajectories and adversely affect driving distance for less skilled players. For the lower skill level players, slicing and hooking the ball are constant obstacles. When a club head strikes a ball improperly, an unintentional side spin is often imparted to the ball, which sends the ball off its intended course. The side spin reduces a player's control over the ball, as well as the direct-line distance the ball will travel. A golf ball that spins less tends not to drift off-line erratically if the ball is not hit squarely with the club face. A low spin ball will not cure the hook or slice, but will reduce the adverse effects of the side spin. Hence, recreational players typically prefer a golf ball that exhibits low spin rate.
In general, the spin rate of golf balls is the end result of many construction variables including the distribution of the density or specific gravity within the golf ball and the relative compression among the various layers in multi-layer golf balls.
Varying materials, density or specific gravity among the various layers of a golf ball controls the spin rate of the golf ball. In one instance, weight from the outer layers of the golf ball is redistributed to the inner layers to decrease the moment of inertia of the golf ball, thereby increasing the spin rate. For example, U.S. Pat. No. 4,625,964 discloses a golf ball with a reduced moment of inertia having an inner core with specific gravity of at least 1.50 and a diameter of less than 32 mm and an intermediate layer of lower specific gravity between the inner core and the cover. U.S. Pat. No. 5,104,126 discloses a ball with a dense inner core having a specific gravity of at least 1.25 encapsulated by a lower density syntactic foam composition. U.S. Pat. No. 5,048,838 discloses another golf ball with a dense inner core having a diameter in the range of 15-25 mm with a specific gravity of 1.2 to 4.0 and an outer layer with a specific gravity of 0.1 to 3.0 less than the specific gravity of the inner core. U.S. Pat. No. 5,482,285 discloses another golf ball with reduced moment of inertia by reducing the specific gravity of an outer core to 0.2 to 1.0.
The total weight of a golf ball has to conform to weight limits set by the United States Golf Association (“USGA”). Although the total weight of the golf ball is controlled, the distribution of weight within the ball can vary. Redistributing the weight or mass of the golf ball either toward the center of the ball or toward the outer surface of the ball changes the dynamic characteristics of the ball at impact and in flight. Specifically, if the density is shifted or redistributed toward the center of the ball, the moment of inertia of the golf ball is reduced, and the initial spin rate of the ball as it leaves the golf club increases as a result of lower resistance from the ball's moment of inertia. Conversely, if the density is shifted or redistributed toward the outer surface of the ball, the moment of inertia is increased, and the initial spin rate of the ball as it leaves the golf club would decrease as a result of the higher resistance from the golf ball's moment of inertia.
The redistribution of weight within the golf ball is typically accomplished by adding fillers to one or more of the core or cover layers of the golf ball. Conventional fillers include the high specific gravity fillers, such as metal or metal alloy powders, metal oxide, metal searates, particulates, and carbonaceous materials and low specific gravity fillers, such as hollow spheres, microspheres and foamed particles. However, the addition of fillers may adversely interfere with the resiliency of the polymers used in golf balls and thereby the coefficient of restitution of the golf balls.
Current dual core golf ball compositions include a stiffening agent in the outer core layer surrounding the inner core. The resulting core has an increased compression. Harder cores, however, produce higher spin rates. Hence, a need remains for a golf ball having a low compression core and increased initial speed.